Temperature measurement apparatus and method

ABSTRACT

A temperature measurement apparatus includes a light source; a first splitter that splits a light beam into a measurement beam and a reference beam; a reference beam reflector that reflects the reference beam; an optical path length adjustor; a second splitter that splits the reflected reference beam into a first reflected reference beam and a second reflected reference beam; a first photodetector that measures an interference between the first reflected reference beam and a reflected measurement beam obtained by the measurement beam reflected from a target object; a second photodetector that measures an intensity of the second reflected reference beam; and a temperature calculation unit. The temperature calculation unit calculates a location of the interference by subtracting an output signal of the second photodetector from an output signal of the first photodetector, and calculates a temperature of the target object from the calculated location of the interference.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 12/399,431, filed on Mar. 6, 2009, the entire contents of which isincorporated herein by reference. U.S. application Ser. No. 12/399,431claims the benefit of priority to Japanese Patent Application No.2008-059027, filed on Mar. 10, 2008 and also claims the benefit of U.S.Provisional Application No. 61/050,650 filed on May 6, 2008.

FIELD OF THE INVENTION

The present invention relates to a temperature measuring apparatus and atemperature measuring method capable of accurately measuring atemperature of, e.g., an a front surface, a rear surface and an innerlayer of an object such as a semiconductor wafer, a liquid crystaldisplay substrate and the like.

BACKGROUND OF THE INVENTION

In general, accurate measurement of a temperature of a substrate (e.g.,a semiconductor wafer) to be processed by a substrate processingapparatus is very important in controlling the shape and property of afilm, a hole and the like formed thereon by various processes such asfilm forming, etching and the like. For this reason, various methods ofmeasuring a temperature of a semiconductor wafer, including ameasurement method using a resistance thermometer or a fluorescentthermometer for measuring a temperature of a rear surface of a substrateand the like, have been performed.

In recent years, temperature measurement technology that uses alow-coherence interferometer and enables the direct measurement of thetemperature of a wafer, which was difficult in the conventionaltemperature measurement method, is generally known. The temperaturemeasurement technology using a low-coherence interferometer isconfigured such that light from a light source is split into measurementbeam for temperature measurement and reference beam by a splitter, aninterference between reflected measurement beam and the reference beam,which is reflected from a reference beam reflector provided with adriving mechanism for changing an optical path length, is measured, andtemperature is measured (see, for example, Japanese Patent ApplicationPublication No. 2006-112826).

In the above-described prior art, the temperature of a wafer can bedirectly measured through a simple construction. However, the inventorsfound as a result of detailed investigation that several types of noiseare contained in a reference signal received from the reference beamreflector having the driving mechanism for changing an optical pathlength. For this reason, errors may be caused in detecting the center ofgravity of an interference waveform, thus deteriorating the accuraciesof an optical thickness measurement and a temperature measurement.

SUMMARY OF THE INVENTION

In view of the above, the present invention provides a temperaturemeasurement apparatus and method, which can measure temperature withhigher accuracy than the prior art and can execute substrate processingor the like with higher accuracy and higher efficiency.

In accordance with a first aspect of the present invention, there isprovided a temperature measurement apparatus, comprising: a lightsource; a first splitter that splits a light beam emanated from thelight source into a measurement beam and a reference beam; a referencebeam reflector that reflects the reference beam; an optical path lengthadjustor that adjusts an optical path length of the reference beamreflected from the reference beam reflector; a second splitter thatsplits the reflected reference beam into a first reflected referencebeam and a second reflected reference beam; a first photodetector thatmeasures an interference between the first reflected reference beam anda reflected measurement beam obtained by the measurement beam reflectedfrom a target object; a second photodetector that measures an intensityof the second reflected reference beam; and a temperature calculationunit that calculates a location of the interference by subtracting anoutput signal of the second photodetector from an output signal of thefirst photodetector, and calculates a temperature of the target objectfrom the calculated location of the interference.

Further, in accordance with a second aspect of the present invention,there is provided a temperature measurement apparatus, comprising alight source; a first splitter that splits a light beam emanated fromthe light source into a measurement beam and a reference beam; areference beam reflector that reflects the reference beam; an opticalpath length adjustor that adjusts an optical path length of thereference beam reflected from the reference beam reflector; a secondsplitter that splits the reflected reference beam into a first reflectedreference beam and a second reflected reference beam; a firstphotodetector that measures an interference between the first reflectedreference beam and a reflected measurement beam obtained by themeasurement beam reflected from a target object; a second photodetectorthat measures an intensity of the second reflected reference beam; asubtraction unit that subtracts an output signal of the secondphotodetector from an output signal of the first photodetector; and atemperature calculation unit that calculates a location of theinterference from a signal obtained by the subtraction of thesubtraction unit, and calculates a temperature of the target object fromthe calculated location of the interference.

Further, in accordance with a third aspect of the present invention,there is provided a temperature measurement apparatus, comprising alight source; a first splitter that splits a light beam emanated fromthe light source into a measurement beam and a reference beam; areference beam reflector that reflects the reference beam; an opticalpath length adjustor that adjusts an optical path length of thereference beam reflected from the reference beam reflector; aphotodetector that measures an interference between the reflectedreference beam and a reflected measurement beam obtained by themeasurement beam reflected from a target object; a shutter unit thatelectively enables incidence of the reflected measurement beam onto thephotodetector; and a temperature calculation unit that stores as areference signal an intensity change of the reflected reference beamwhen the reflected measurement beam is not incident onto thephotodetector by closing the shutter unit, calculates a location of theinterference from a signal obtained by subtracting the reference signalfrom an output signal of the photodetector when the reflectedmeasurement beam is incident onto the photodetector, and calculates atemperature of the target object from the calculated location of theinterference.

Further, in accordance with a fourth aspect of the present invention,there is provided a temperature measurement apparatus, comprising alight source; a first splitter that splits a light beam emanated fromthe light source into a measurement beam and a reference beam; areference beam reflector that reflects the reference beam; an opticalpath length adjustor that adjusts an optical path length of thereference beam reflected from the reference beam reflector; aphotodetector that measures an interference between the reflectedreference beam and a reflected measurement beam obtained by themeasurement beam reflected from a target object; a filter that filtersan output signal of the photodetector according to a frequency thereof;and a temperature calculation unit that calculates a location of theinterference from the signal filtered by the filter, and calculates atemperature of the target object from the calculated location of theinterference.

In the above configuration, it is preferable that the filter is ananalog filter or a digital filter.

In the above configuration, it is also preferable that the filter cutsoff a frequency component whose frequency is lower than that of a noisecaused by operation of the optical path length adjustor.

In the above configuration, it is also preferable that the filter allowsa frequency component whose frequency is higher than that of aninterference wave between the reflected measurement beam and thereflected reference beam to pass therethrough.

In the above configuration, it is also preferable that the filter passesallows only a frequency band component whose frequency is equal to thatof an interference wave between the reflected measurement beam and thereflected reference beam to pass therethrough.

Further, in accordance with a fifth aspect of the present invention,there is provided a temperature measurement method, comprising radiatinga reference beam onto reference beam reflector while radiating ameasurement beam onto a target object; measuring an interference betweenthe reference beam reflected from the reference beam reflector and themeasurement beam reflected from the target object while changing anoptical path length of the reflected reference beam by moving thereference beam reflector in one direction; and subtracting a signal,obtained when only the reflected reference beam is detected while theoptical path length of the reflected reference beam is changed, from asignal obtained from the interference measurement, calculating alocation of the interference by the subtraction, and calculating atemperature of the target object from the calculated location of theinterference.

Further, in accordance with a sixth aspect of the present invention,there is provided a temperature measurement method, comprising radiatinga reference beam onto reference beam reflector while radiating ameasurement beam onto a target object; measuring an interference betweenthe reference beam reflected from the reference beam reflector and themeasurement beam reflected from the target object while changing anoptical path length of the reflected reference beam by moving thereference beam reflector in one direction; and calculating a location ofthe interference from a signal obtained by filtering an output signal ofthe interference measurement according to a frequency, and calculating atemperature of the target object from the calculated location of theinterference.

With the above configurations, the present invention can provide atemperature measurement apparatus and method which can measuretemperature with higher accuracy than that in the prior art, and canexecute substrate processing or the like with higher accuracy and higherefficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

The above features of the present invention will become apparent fromthe following description of embodiments, given in conjunction with theaccompanying drawings, in which:

FIG. 1 is a block diagram showing the schematic construction of atemperature measurement apparatus according to a first embodiment of thepresent invention;

FIGS. 2A and 2B illustrate pictures of interference waveforms betweenreflected measurement beam and reflected reference beam;

FIG. 3 illustrates an enlarged picture showing the interference waveformbetween the reflected measurement beam and the reflected reference beamand the waveform of the reflected reference beam;

FIG. 4 is a block diagram showing the schematic construction of atemperature measurement apparatus according to a second embodiment ofthe present invention;

FIG. 5 is a block diagram showing the schematic construction of atemperature measurement apparatus according to a third embodiment of thepresent invention;

FIG. 6 is a block diagram showing the schematic construction of atemperature measurement apparatus according to a fourth embodiment ofthe present invention; and

FIG. 7 is a block diagram showing the schematic construction of atemperature measurement apparatus according to a fifth embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the attached drawings. Further, in the presentspecification and drawings, the same reference numerals are used todesignate components having substantially identical functions, and theirrepeated descriptions will be omitted.

FIG. 1 illustrates the schematic construction of a temperaturemeasurement apparatus 100 according to a first embodiment of the presentinvention. As shown in FIG. 1, the temperature measurement apparatus 100includes a light source 110, a first splitter 120, a reference beamreflector 140, an optical path length adjustor 150, and a secondsplitter 121. The first splitter 120 splits light emitted from the lightsource 110 into a measurement beam for temperature measurement and areference beam. The reference beam reflector 140 reflects the referencebeam received from the first splitter 120. The optical path lengthadjustor 150 changes the optical path length of the reference beamreflected from the reference beam reflector 140. The second splitter 121splits the reflected reference beam from the reference beam reflector140 into two parts.

The optical path length adjustor 150 includes a linear stage 151, amotor 152, and a He—Ne laser encoder 153 which are configured to movethe reference beam reflector 140 such as a reference mirror in onedirection parallel to the incident direction of the reference beam. Theoptical path length of the reference beam reflected from the referencemirror can be adjusted by driving the reference mirror in one direction.The motor 152 is controlled by a controller 170 through a motorcontroller 155 and a motor driver 154. Further, a signal from the He—Nelaser encoder 153 is converted into a digital signal by anAnalog/Digital (A/D) converter 172, and is then input to the controller170.

Further, the temperature measurement apparatus 100 includes a firstphotodetector 160 configured to measure an interference between themeasurement beam reflected from a target object 10 such as asemiconductor wafer obtained when the measurement beam is radiated ontothe target object 10, and one of two reflected reference beams split bythe second splitter 121 after the reference beam is radiated onto andreflected from the reference beam reflector 140. Further, thetemperature measurement apparatus 100 has a second photodetector 161 formeasuring the intensity of the remaining one of the two reflectedreference beams split by the second splitter 121.

The kind of light adopted by the light source 110 is not particularlylimited, so long as the interference between the measuring beams and thereference beam can be measured. When the temperature measurement of thesemiconductor wafer as the temperature measurement object 100 isperformed, the light may preferably be chosen such as not to cause aninterference between two reflected beams reflected respectively at afront surface of the semiconductor wafer and a rear surface of thesemiconductor wafer (the distance therebetween is, typically, about 800to 1500 μm).

Specifically, it is preferable to use a low coherence light. The lowcoherence light is a kind of light having a short coherence length. Forexample, a center wavelength of the low coherence light may preferablybe 0.3 to 20 μm; and more preferably, 0.5 to 5 μm. Further, thecoherence length may preferably be 0.1 to 100 μm; and more preferably, 3μm or less. By using the low coherence light for the light source 110,it is possible to avoid problems due to the presence of unwantedinterference, and it becomes easier to measure the interference betweenthe reference beam and the measurement beam reflected from a surface oran inner layer of the semiconductor wafer.

The light source using the low coherence light may be a superluminescent diode (SLD), an light emitting diode (LED), a highbrightness lamp (a tungsten lamp, a xenon lamp and the like), anultra-wideband wavelength light source or the like. Among these lowcoherence light sources, an SLD having a high brightness (whosewavelength is, for example, 1300 nm) may preferably be used as the lightsource 110.

A 2×2 optical fiber coupler may be used as the first splitter 120.However, the first splitter 120 is not limited thereto, and it ispossible to use any device capable of splitting light into a referencebeam and a measurement beam. Further, a 2×1 optical fiber coupler may beused as the second splitter 121. However, the second splitter 121 is notlimited thereto, and it is possible to use any device capable ofsplitting reflected reference beam into two parts. Alternatively, anoptical waveguide demultiplexer, a semi-transparent mirror and the likemay also be employed as the first splitter 120 and the second splitter121.

The reference beam reflector 140 is implemented using, for example, areference mirror. For example, a corner cube prism or a planar mirrormay be used as the reference mirror. Of these, the corner cube prism ispreferably used from the standpoint of the parallelism of reflectedlight with incident light. However, the reference beam reflector 140 isnot limited to this example, and may be implemented using any devicecapable of reflecting reference beam, for example, a delay line.

As the first and the second photodetectors 160 and 161, photodiodes maybe used in consideration of a low price and a good compactness.Specifically, each of the photodetectors 160 and 161 may be formed of aphotodetector (PD) using, for example, a Si photodiode, an InGaAsphotodiode, a Ge photodiode and the like. However, without being limitedthereto, the photodetector 160 may be constituted of other devices suchas an avalanche photodiode, a photomultiplier tube and the like. Thedetection signal detected by the first photodetector 160 is input to theA/D converter 172 via an amplifier 171, then is converted into a digitalsignal, and is processed by the controller 170. The detection signaldetected by the second photodetector 161 is input to the A/D converter172 via an amplifier 173, and is converted into a digital signal, and isprocessed by the controller 170.

The measurement beam from the first splitter 120 is to be transmitted toa measurement beam radiating position, at which the measurement beam isradiated onto the target object 10, via a measurement beam transmissionmeans such as a collimator fiber 180. Further, the reference beam fromthe first splitter 120 passes through the second splitter 121 to betransmitted to a reference beam radiating position, at which thereference beam is radiated onto the reference beam reflector 140, via areference beam transmission means such as a collimator fiber 190.Further, the measurement beam transmission means and the reference beamtransmission means are not limited to the collimator fibers, and may beimplemented as, for example, optical fibers each equipped with acollimator in such a way that the collimator is attached to the front ofthe optical fiber.

In the temperature measurement apparatus 100, light from the lightsource 110 is incident on the first splitter 120 and is split into ameasurement beam and a reference beam by the first splitter 120. Amongthese, the measurement beam is radiated onto the target object 10 suchas a semiconductor wafer, and is reflected at the front surface or therear surface of each layer, or the interfaces between the respectivelayers.

Meanwhile, the reference beam is reflected at the reference beamreflector 140, and the reflected reference beam is incident on thesecond splitter 121 to be split into two reflected reference beams.Among these, one of the reflected reference beams is incident on thefirst splitter 120, and is detected by the first photodetector 160together with the reflected measurement beam. The other of the reflectedreference beams is detected by the second photodetector 161.

Further, by scanning the reference beam reflector 140 by the opticalpath length adjustor 150, an interference waveform shown in FIG. 2A, inwhich the vertical axis indicates the output level (V) (i.e., theintensity of light) and the horizontal axis indicates the measurementdistance (μm) of the reference beam reflector 140, is obtained by thefirst photodetector 160.

Here, the light source 110 is implemented by the above-describedlow-coherence light source. Since the coherence length of the lightemanated therefrom is short, a strong interference occurs at suchlocations where the optical path length of the measurement beam isequivalent to that of the reference beam, and interference hardly occursat the other locations. For this reason, by moving the reference beamreflector 140 to vary the optical path length of the reference beam, thereflected reference beam interferes with the reflected measurement beamreflected from a front and a rear surface of the temperature measurementobject 10, and, if inner layers exist therein, from each of the innerlayers due to a difference in refractive index.

In the example shown in FIG. 2A, the following sequence is observed asthe reference beam reflector 140 is scanned. First, an interference waveappears by the interference between the reflected beam from one surface(the front surface or the rear surface) of the target object 10 andreflected reference beam. Then, another interference wave appears by theinterference between the reflected beam from the interface ofintermediate layers and the reflected reference beam. Finally, anotherinterference wave appears by the interference between the reflected beamfrom the other surface (the rear surface or the front surface) andreflected reference beam.

As shown in FIG. 2A, a signal waveform detected by the firstphotodetector 160 contains interference wave component having a highfrequency and a high peak as well as other components whose frequenciesare lower than that of the interference wave, and the base line of thesignal waveform undulates noticeably regardless of the interferencewave.

FIG. 3 illustrates an enlarged view of the detected signal waveform. Ofthe two waveforms shown in FIG. 3, the upper waveform is a signalwaveform detected by the first photodetector 160. As shown therein, thecomponents whose frequencies are lower than that of the interferencewave are not single in number, and may contain other components havinghigher frequencies as well as those observable in FIG. 2A. Whencomponents other than the above-described interference wave arecontained in the detected signal waveform, an error may occur whenobtaining the location of interference based on the center of gravity ofthe interference wave.

In FIG. 3, the lower waveform is of the reflected reference beamdetected by the second photodetector 161. It can be seen therefrom thatthe components other than the interference wave contained in the signalwaveform detected by the first photodetector 160 are originated by thereflected reference beam. Such components are thought to be caused byminute mechanical distortion or a clearance, etc., occurring when thereference beam reflector 140 is scanned by the linear stage 151 in theoptical path length adjustor 150.

In view of the above, the present embodiment is configured such that thecontroller 170 receives (via the amplifier 173 and the A/D converter172) not only the detected signal coming from the first photodetector160 but also the detected signal coming from the second photodetector161, which is obtained at the second photodetector 161 by detecting oneof two reflected reference beams output from the second splitter 121.Then, the controller 170 subtracts the signal detected by the secondphotodetector 161 from the signal detected by the first photodetector160.

The detected signal waveform obtained after this subtraction is shown inFIG. 2B. As shown therein, the detected signal waveform obtained bysubtracting the signal detected by the second photodetector 161 from thesignal detected by the first photodetector 160 does not contain thenoise component having occurred when the reference beam reflector 140 isscanned by the optical path length adjustor 150. Therefore, on the basisof the detected signal waveform, accurate interference location can beobtained with low error based on the center of gravity of theinterference wave; and, on the basis of this interference location,accurate temperature measurement can be performed.

Next, methods of measuring temperature based on an interference wavebetween measurement beam and reference beam are described below.Temperature measurement methods based on the interference wave include atemperature conversion method that uses, for example, a change in anoptical path length corresponding to a temperature change. In thefollowing, a temperature conversion method using a deviation in thelocation of an interference waveform is described.

When the target object 10 such as the semiconductor wafer is warmed by aheater or the like, the target object 10 is expanded and the refractiveindex thereof changes. Therefore, the location of an interferencewaveform is different before and after the temperature change, and thedistance between the peaks of the interference waveform also changes.The temperature change can be detected by measuring the distance betweenthe peaks of the interference waveform. In the case of, e.g., thetemperature measurement apparatus 100 of FIG. 1, the distance betweenthe peaks of the interference waveform corresponds to the displacementof the reference beam reflector 140. Therefore, the temperature changecan be detected by measuring the displacement of the reference mirrorcorresponding to the distance between the peaks of the interferencewaveform.

Assuming that the thickness of the target object 10 is d and therefractive index thereof is n, a difference in the peak position of theinterference waveform mainly depends on a linear expansion coefficient αof each layer in regard to the thickness d, and on a temperaturecoefficient β of refractive index change of each layer in regard to thechange in the refractive index n. Further, it is also known that thedifference in the peak position of the interference waveform alsodepends on the wavelength in relation to the temperature coefficient β.

Therefore, the thickness d′ of the wafer at an arbitrary measurementpoint after the temperature change can be represented by Equation (1)shown below. In Equation (1), ΔT is the temperature change at themeasurement point, α is the linear expansion coefficient, and β is thetemperature coefficient of refractive index change. Further, d and n area thickness and a refractive index, respectively, at the measurementpoint before the temperature change.

d′=d·(1+αΔT), n′=n·(1+βΔT)  (1)

As shown in Equation (1), the optical path length of the measurementbeam passing through the measurement point varies due to temperaturechange. The optical path length is generally represented by a product ofthe thickness d and the refractive index n. Therefore, assuming that theoptical path length of the measurement beam passing through themeasurement point before the temperature change to be L, and that theoptical path length at the measurement point after the temperature hasvaried by ΔT to be L′, L and L′ are given as shown in the followingEquation (2).

L=d·n, L′=d′·n′  (2)

Therefore, by calculating the difference L-L′ between the optical pathlengths of the measurement beam at the measurement point before andafter the temperature change by using Equations (1) and (2), thefollowing Equation (3) is obtained. In Equation (3), a negligible termis omitted in consideration of the relationships α·β

α and α·β

β.

L′−L=d′·n′−d·n=d·n·(α+β)·ΔT ₁  (3)

Here, the optical path length of the measurement beam at the measurementpoint corresponds to the distance between the peaks of an interferencewaveform generated by interference between the measurement beam and thereference beam. Therefore, when the linear expansion coefficient α andthe temperature coefficient β are known in advance, the distance betweenthe peaks of the interference waveform generated by interference betweenthe measurement beam and the reference beam at the measurement point canbe converted into the temperature at the measurement point by measuringthe distance between the peaks and applying Equation (3).

In case of interpreting the interference wave into the temperature inthe above-discussed manner, it should be noted that the optical pathlength appearing between the peaks of the interference waveform changesdue to the linear expansion coefficient α and the temperaturecoefficient β as described above, and thus the coefficients α and β needto be obtained in advance.

Generally, the linear expansion coefficient α and the temperaturecoefficient β of a given material including a semiconductor wafer may bedependent upon the temperature, depending on the temperature range.Since, for example, the linear expansion coefficient α does not changegreatly in a temperature range from 0 to 100° C., it may be regarded asa constant. However, in a temperature range of higher than 100° C., therate of change may increase as temperature increases, depending on thekind of material. In this case, the temperature dependence cannot benegligible. Similarly, the temperature dependence of the temperaturecoefficient β may also become unnegligible, depending on the temperaturerange.

In the case of, e.g., silicon (Si) that is used as a main component ofthe semiconductor wafer, the linear expansion coefficient α and thetemperature coefficient β can be approximated as, for example, aquadratic curve in a temperature range from 0 to 500° C. As describedabove, the linear expansion coefficient α and the temperaturecoefficient β depend on temperature. Accordingly, by obtaining inadvance the coefficients α and β as functions of the temperature, andperforming temperature conversion based on the values of thecoefficients, more accurate temperatures can be acquired.

Further, the temperature measurement method based on an interferencewave between a measurement beam and a reference beam are not limited tothe above, and may make use of a change in absorption intensity due to atemperature change or combine a change in optical path length and achange in absorption intensity due to a temperature change.

FIG. 4 is a diagram showing a temperature measurement apparatus 100 aaccording to a second embodiment of the present invention. Thetemperature measurement apparatus 100 a is configured such that theoutputs of a first photodetector 160 and a second photodetector 161 areinput to a differential circuit 174, and the difference therebetween isobtained (by subtraction). Then, the output of the differential circuit174 is input to a controller 170 a via an A/D converter 172. Byobtaining the difference between analog signals using the differentialcircuit 174 in the temperature measurement apparatus 100 a, the sameeffects as those of the first embodiment can be obtained.

FIG. 5 is a diagram showing the construction of a temperaturemeasurement apparatus 100 b according to a third embodiment of thepresent invention. In the temperature measurement apparatus 100 b,instead of providing a second splitter 121 and a second photodetector161 as in the above embodiments, a shutter unit 181 for electivelyenabling or disabling the incidence of a reflected measurement beam ontoa first photodetector 160 is provided, e.g., between the end of acollimator fiber 180 and a target object 10. Further, the firstphotodetector 160 is configured to detect only the reference beamreflected from the reference beam reflector 140 by closing the shutterunit 181.

The temperature measurement apparatus 100 b detects only a reflectedreference beam from the reference beam reflector 140 by closing theshutter unit 181, and stores thus detected data (which is a waveformdata shown in the lower portion of FIG. 3) in the controller 170 asreference signal data. When an interference between the reflectedmeasurement beam and the reflected reference beam is detected from theoutput signal of the first photodetector 160 by opening the shutter unit181, the controller 170 subtracts the stored reference signal data fromthe output signal of the first photodetector 160.

The temperature measurement apparatus 100 b configured as above can beimplemented at a low cost with a relatively simple construction, withouthaving to provide the second splitter 121 and the second photodetector161 as in the case of the temperature measurement apparatus 100 ofFIG. 1. Further, the temperature measurement apparatus 100 b can measuretemperature with a higher accuracy by eliminating noise componentsoccurring due to the operation of the optical path length adjustor 150.

FIG. 6 is a diagram showing the construction of a temperaturemeasurement apparatus 100 c according to a fourth embodiment of thepresent invention. The temperature measurement apparatus 100 c isconfigured such that the output signal of a first photodetector 160 isinput to an amplifier 171 via a filter 175, and then is input to acontroller 170 via an A/D converter 172. The filter 175 is an analogfilter for eliminating the above-described noise components whosefrequencies are lower than that of an interference wave.

The filter 175 may be configured to perform one of the followings:cutting off frequency components whose frequencies are lower than thoseof noises caused by the operation of the optical path length adjustor150; selectively allowing only such frequency components whosefrequencies are higher than that of an interference wave between areflected measurement beam and a reflected reference beam to passtherethrough; and selectively allowing only such frequency bandcomponents corresponding to the frequency of the interference wavebetween the reflected measurement beam and the reflected reference beamto pass therethrough.

The temperature measurement apparatus 100 c configured as above can beimplemented at a low cost by a relatively simple construction, withouthaving to provide the second splitter 121 and the second photodetector161 as in the temperature measurement apparatus 100 of FIG. 1. Further,the temperature measurement apparatus 100 c can measure temperature withhigher accuracy by eliminating noise components occurring due to theoperation of the optical path length adjustor 150.

FIG. 7 is a diagram showing the construction of a temperaturemeasurement apparatus 100 d according to a fifth embodiment of thepresent invention. The temperature measurement apparatus 100 d isconfigured such that the function of a digital filter is provided in acontroller 170′, instead of providing the filter 175 shown in FIG. 6. Inthis case as well, the same effects as those of the temperaturemeasurement apparatus 100 c using the analog filter of the fourthembodiment can be achieved.

While the invention has been shown and described with respect to thepreferred embodiment, it will be understood by those skilled in the artthat various changes and modifications may be made without departingfrom the scope of the invention as defined in the following claims.

1. A temperature measurement apparatus, comprising: a low coherencelight source; a first splitter that splits a low coherence light beamemanated from the low coherence light source into a measurement beam anda reference beam; a reference beam reflector that reflects the referencebeam; an optical path length adjustor that adjusts an optical pathlength of the reference beam reflected from the reference beamreflector; a second splitter that splits the reflected reference beaminto a first reflected reference beam and a second reflected referencebeam; a first photodetector that measures an interference between thefirst reflected reference beam and a reflected measurement beam obtainedby the measurement beam reflected from a target object; a secondphotodetector that measures an intensity of the second reflectedreference beam; and a temperature calculation unit that calculates alocation of the interference by subtracting an output signal of thesecond photodetector from an output signal of the first photodetector,and calculates a temperature of the target object from the calculatedlocation of the interference.
 2. A temperature measurement apparatus,comprising: a low coherence light source; a first splitter that splits alow coherence light beam emanated from the low coherence light sourceinto a measurement beam and a reference beam; a reference beam reflectorthat reflects the reference beam; an optical path length adjustor thatadjusts an optical path length of the reference beam reflected from thereference beam reflector; a second splitter that splits the reflectedreference beam into a first reflected reference beam and a secondreflected reference beam; a first photodetector that measures aninterference between the first reflected reference beam and a reflectedmeasurement beam obtained by the measurement beam reflected from atarget object; a second photodetector that measures an intensity of thesecond reflected reference beam; a subtraction unit that subtracts anoutput signal of the second photodetector from an output signal of thefirst photodetector; and a temperature calculation unit that calculatesa location of the interference from a signal obtained by the subtractionof the subtraction unit, and calculates a temperature of the targetobject from the calculated location of the interference.
 3. Atemperature measurement method, comprising: splitting a low coherencelight beam into a reference beam and a measurement beam; radiating thereference beam onto a reference beam reflector while radiating themeasurement beam onto a target object; measuring an interference betweenthe reference beam reflected from the reference beam reflector and themeasurement beam reflected from the target object while changing anoptical path length of the reflected reference beam by moving thereference beam reflector in one direction; and subtracting a signal,obtained when only the reflected reference beam is detected while theoptical path length of the reflected reference beam is changed, from asignal obtained from the interference measurement, calculating alocation of the interference by the subtraction, and calculating atemperature of the target object from the calculated location of theinterference.
 4. The temperature measurement apparatus of claim 1,wherein a center wavelength of the low coherence light beam ranges from0.3 μm to 20 μm and a coherence length of the low coherence light beamranges from 0.1 μm to 100 μm.
 5. The temperature measurement apparatusof claim 2, wherein a center wavelength of the low coherence light beamranges from 0.3 μm to 20 μm and a coherence length of the low coherencelight beam ranges from 0.1 μm to 100 μm.
 6. The temperature measurementmethod of claim 3, wherein a center wavelength of the low coherencelight beam ranges from 0.3 μm to 20 μm and a coherence length of the lowcoherence light beam ranges from 0.1 μm to 100 μm.